It has been suggested that acrosin, the trypsin-like acrosomal enzyme of mammalian sperm, plays a role in the penetration of the mammalian egg zona pellucida. Our results have demonstrated the existence of proacrosin, a zymogen precursor of acrosin in rabbit testes and sperm and hamster sperm. The objectives of this research are the elucidation of the molecular nature of proacrosin and the elucidation of the mechanism of proacrosin activation.